Flash-butt welding or “flash welding” is a resistance welding technique for joining segments of metal rail, rod, chain or pipe in which the segments are aligned end to end and heated by electrical currents, producing an electric arc that melts and welds the ends of the segments, yielding an exceptionally strong and smooth joint.
A flash butt welding circuit usually consists of a low-voltage, high-current energy source (usually a welding transformer) and two clamping electrodes. The two segments that are to be welded are clamped in the electrodes and brought together until they meet, making light contact. Energizing the transformer causes a high-density current to flow through the areas that are in contact with each other. Flashing starts, and the segments are forged together with sufficient force and speed to maintain a flashing action. After a heat gradient has been established on the two edges to be welded, an upset force is applied to complete the weld. This upset force extrudes slag, oxides and molten metal from the weld zone leaving a welding accretion in the colder zone of the heated metal. The joint is then allowed to cool slightly before the clamps are opened to release the welded article. The welding accretion may be left in place or removed by shearing while the welded article is still hot or by grinding, depending on the requirements.
International publication no. WO 2006/103021 discloses a welded roller bearing ring made from a cold-rolled profile wire of roller bearing steel having a hypereutectic composition and a carbon content of at least 0.7%. The welded roller bearing ring comprises a soft-annealed coarse-grained nodular cementite joint obtained by butt welding. The region around the welded joint comprises a martensitic cementite structure with a higher carbide number and a finer structure relative to the remaining ring region. During the flash butt welding process, when two surfaces are forged together, a material flow perpendicular to the plane of the two surfaces is created. This material flow forms a grain structure or fibre flow oriented perpendicular to the plane of the two surfaces. Inclusions present within the material become incorporated in this material flow.
In conventional bearing steels, the dominating inclusion type is sulphides due to the fact that the sulphur content normally is higher than the oxygen content. Since sulphides have an elongated shape they can become highly oriented during flash butt welding and thus make the steel anisotropic in the area of the weld joint. It has been found that the life time of a bearing component is most adversely affected by oxygen-containing inclusions, such as sulphide inclusions containing encapsulated or embedded oxide inclusions, since when such inclusions have matrix contact, they act as crack initiators. When a component such as a bearing ring is being flash butt welded, the resulting fibre flow carrying incorporated sulphides will therefore be unfavourable with respect to fatigue crack initiation and propagation in the finished welded bearing ring compared to a bearing that does not comprise a flash butt weld joint. In addition, the sulphides in bearing steels can be fully or partially dissolved in austenite in the weld zone. On cooling, the sulphides will preferentially precipitate at grain boundaries which will significantly weaken the weld zone.
In order to avoid these problems with sulphides during flash butt welding it is not advantageous to reduce the sulphur content of the bearing steel to as close to zero as possible since this results in magnesium and calcium in the melt entering oxide inclusions in the form of aluminates and forming undesired complex aluminate inclusions. Pure aluminates are hard and brittle; they will break during hot forming and do not therefore pose a substantial problem to manufacturers of bearing components with high degrees of forming deformation. However, complex aluminates can be hard but they are not brittle so they will remain intact during rolling and will therefore be incorporated into the finished bearing component. If a complex aluminate inclusion becomes located in an area of the bearing component subject to heavy loading, this is where a fatigue failure will start.